Flow Cytometric immunophenotyping is a sensitive technique for analysis of benign and malignant tumors. We are studying the refinement of this technique and its application to diagnosis and measurement of prognostic markers in different systems. We have developed methods to quantitate antibody binding capacity in tumor cells present in low levels in lymph node, peripheral blood or bone marrow for patients under going antibody based therapy. This flow cytometric assay is performed in place of previous studies using radioactively labeled antibodies to measure tumor cell binding capacity in patients. The flow cytometric assay is more rapid, makes the use of radioactivity unnecessary and is performed on 100 mL of blood instead of 25mL. Furthermore, the data is more specific as levels of antibody binding are measured specifically in the tumor cells without the influence of other populations present in patient specimens. The data collected is also more precise and will be used to compare antibody binding to tumor cells to treatment response. Recent studies, using flow cytometry have demonstrated that endothelial cell precursors as well as mature endothelial cells can be detected in the circulation of patients with cancer and that the levels of these cells are significantly higher in patients with malignancy than normal controls. Furthermore, successful therapy for the malignant process may affect the number of activated circulating endothelial cells (CEC's) in the peripheral blood. These data suggest that measurements of CEC's and circulating endothelial cell precursors (CEP's) in the peripheral blood of cancer patients may be a used as a surrogate marker for the efficacy of anti-cancer strategies, particularly those that target tumor vasculature. We have developed methodology to measure levels of CECs and CEPs in the peripheral blood of patients with cancer or of patients undergoing surgical interventions, and to determine if these measurements can be used to follow response or disease progression across a variety of histologies and treatments.